A 10-year-old girl is brought to the clinic by her father because she has had pain in her right upper leg for the past 3 months. On physical examination, the patient is unable to bear weight on the limb. Edema of the upper leg and tenderness to palpation are noted. X-rays show 2 bone lesions with sharp margins and a punched-out appearance. Which of the following is the most appropriate definitive step in diagnosis?